1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) multi-user receiving apparatus including an interference canceller, and more particularly to a CDMA multi-user receiving apparatus including an interference canceller which an interference removing process is carried out at each of stages to output demodulation signals from all users.
2. Description of the Related Art
Conventionally, a CDMA receiving apparatus spreads is used for a communication system using a CDMA system, in which an information signal is spread and modulated with the signal ten times to hundred times faster than the information signal.
For such a CDMA receiver, a single user receiver is generally used to receive a signal from one user. However, it is requested in recent years to efficiently extract a desired user signal, while removing interference with signals from other users. In this case, the interference changes depending upon time.
To respond to this request, a multi-user type interference canceller is known as described in, for example, the technical report of the electronic, information, and communication engineers, RCS 96-100. In the reference, replicas (spread signals) of other user signals are reproduced from a reception signal by use of a determined symbol, a transmission path data and a spreading code, and the reproduced spread signals are subtracted from the reception signal. Then, a desired user signal is demodulated by use of the subtracted signal once again.
The technique described in the above reference is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 10-51353), and will be described with reference to a block diagram of the CDMA multi-user receiving apparatus shown in FIG. 1.
Referring to FIG. 1, the receiving apparatus is composed of three receiving apparatus stage 1 to 3. In each of the receiving apparatus stages 1 to 3, a demodulating process and an interference removal process are carried out regardless of the reception signal level. Each of the receiving apparatus stages 1 and 2 is composed of three interference estimating units (IEUs) 101-1 to 101-6, multipliers 102-1 to 102-6, delay units 106-1 and 106-2 and adders 107-1 and 107-2. Each of the interference estimating units 101-1 and 101-4, 101-2 and 101-5, and 101-3 and 101-6 corresponds to 3 user signals. Each of the delay units 106-1 and 106-2 corresponds to the multipliers 102-1 to 102-3, or 102-4 to 102-6. Each of the adders 107-1 and 107-2 corresponds the delay unit 106-1 or 106-2. The adders 107-1 and 107-2 input the outputs of the delay units 106-1 and 106-2 and output an adding result to three interference estimating units 101-4 to 101-6, and 101-7 to 101-9. The respective interference estimating units 101-1 to 101-3, 101-4 to 101-6, and 101-7 to 101-9 at each stage input a reception signal and interference removal residual signals obtained from the adders 107-1 and 107-2 at the previous stage, respectively. Also, the interference estimating units 101-4 to 101-6 and 101-7 to 101-9 input reception symbol replicas of the corresponding user signals which have been estimated in the interference estimating units 101-1 to 101-3 and 101-4 to 101-6 at the previous stage. The interference estimating units 101-1 to 101-3 and 101-4 to 101-6 re-estimate the reception symbol replicas of the current stage to output to the interference estimating units 101-4 to 101-6 and 101-7 to 101-9 for the same user signals at the next stage. Also, the interference estimating units 101-1 to 101-3, and 101-4 to 101-6 output spreading process results of the difference between the reception symbol replicas at the current stage and the reception signal or the interference removal residual signals (hereinafter, to be referred to as error signals).
In each of the interference estimating units 101-7 to 101-9 at the last stage 3, it is not necessary to re-estimate interference replicas at the current stage 3. Therefore, demodulation result are outputted just as they are, as demodulation signals A to C for the respective user signals.
The interference removal residual signal inputted to each of the interference estimating units 101-1 to 101-3 at the first stage 1 is the reception signal itself. At the last stage 3, the delay unit 106 and the adder 107 are omitted.
The delay unit 106 delays the interference removal residual signal which is also supplied to the interference estimating unit 101 by the time until a reproduction signal is outputted from the multiplier 102. The delayed signal is supplied to the adder 107. The error signal to be supplied to the adder 107 is obtained by multiplying a weighting factor xcex1 with the output of the interference estimating unit 101. The interference removal residual signal is obtained by subtracting three error signals of the interference estimating units 101 from the output of the delay unit 106 and is outputted to the next stage.
In this way, when there are many user signals which are to be processed in parallel so that the characteristic sometimes degrades, the reliability of the interference removal residual signal obtained through the interference removing process is sometimes decreased, if the multiplication of the interference removal suppression weighting coefficient xcex1 is carried out. Therefore, the weighting coefficient xcex1 below xe2x80x9c1xe2x80x9d is multiplied to prevent the decrease of the reliability of the interference removal residual signal and the degradation of the interference removal characteristic.
However, as seen from the above description, the interference removal suppression coefficient xcex1 is fixed. Therefore, there is a problem in that the interference removal suppression coefficient xcex1 is not always optimal to the number of user signals which can be transmitted at the same time, when the receiving states of the spread signal change randomly. Consequently, there is a problem in that the change of the operation point of the interference removal suppression coefficient xcex1 prevents the convergence operation of a multi-user type interference canceller so that the operation of the interference canceller diverges.
Also, there is another problem in that the interference removal suppression coefficient xcex1 is initially set to a small value so that the ability of the interference removing process can be suppressed to a low level, in order to operate the interference canceller in a stable state under any reception situation.
In conjunction with the above description, a receiving apparatus is described in Japanese Laid Open Patent Application (JP-A-Heisei 7-273713). In this reference, a sync signal is detected by a sync signal detecting section (10) and amplitude information (A1 to A3) and phase information (xcex81 to xcex83) of a main wave and a delay wave are generated from the reproduced sync signal. A sync signal removing section (9) removes the sync signal from a signal supplied from a radio demodulating section (8) by use of the reproduced sync signal. A reception signal r obtained thus is supplied to an other station interference removing sections (11). The other station interference removing sections (11 to 1K) estimate the first to N-th station signal while removing the interference between the stations from the reception signal by use of the amplitude information (A1 to A3) and the phase information (xcex81 to xcex83). A correlation detecting operation is performed to the estimated signals (r1K to rNK) by correlation detecting section (21 to 2N). Thus, the demodulation signals (S1 to SN) are obtained for the first to N-th stations.
Also, a DS-CDMA interference canceller is described in Japanese Laid Open Patent Application (JP-A-Heisei 9-331274). In this reference, a reception signal (S1) is supplied to a receivers (21 to 2n) and a reception level is measured for each user signal in units of blocks. A receiving level raking calculating section (18c) calculates the ranking of the reception level from the measuring result. The reception signal (S1) is supplied to a receiving signal blocking section (18a) which groups the reception signal into blocks. A ranking information adding section (18b) performs the ranking of the receiving levels inputted from the receiving level ranking calculating section (128c). ICUs (13-1 to 14-n) refers the receiving level ranking information of the reception signal blocks to perform an interference canceling operation to the reception signal for the corresponding user. Thus, demodulation signals are outputted from decoder (16-1 to 16-n).
Also, a DS-CDMA multi-user serial interference canceller is described in Japanese Patent No. 2737775. In this reference, the DS-CDMA multi-user serial interference canceller is composed of first to N-th (N is a natural number equal to or more than 2) stages, each of which includes k interference canceling units (k is a natural number equal to or more than 1). Each of users at each of the stages is transferred with a summation of interference replica signals of the other users which are produced in previous to a current stage. An interference replica signal of the user which is produced in the previous stage is subtracted from the summation of interference replica signals of the other users to produce a residual interference replica signal. The residual interference replica signal is subtracted from a reception signal and the subtracting result is supplied to the interference canceling unit of the user.
An object of the present invention is to provide a multi-stage type interference canceller and a receiving apparatus including it, in which an interference removal suppression coefficient is dynamically controlled in correspondence to the change of a reception situation so that stable and high interference removal ability can be accomplished.
Another object of the present invention is to provide a multi-user type interference canceller using an interference removal suppression coefficient in which an interference removal suppression coefficient is dynamically controlled based on a reception signal level such that it becomes possible to perform an optimal reception.
In order to achieve an aspect of the present invention, a multi-user CDMA (Code Division Multiple Access) interference canceller includes a coefficient output section and a plurality of stages. The coefficient output section dynamically determines a weight coefficient based a reception state of each of a plurality of user signals of a reception signal. Each of the plurality of stages other than a last stage includes a first interference estimating section and a synthesizing unit. The first interference estimating section for each of the plurality of user signals receives the weight coefficient for a corresponding user signal from the coefficient output section. Also, the first interference estimating section receives as a stage input signal, the reception signal when the stage is a first stage and a residual signal outputted from a previous stage when the stage is not the first stage. Also, the first interference estimating section receives a symbol replica outputted from the previous stage when the stage is not the first stage. Then, the first interference estimating section outputs the symbol replica for a next stage and a user reproduction signal for the corresponding user signal based on the symbol replica received from the previous stage, the stage input signal and the weight coefficient for the corresponding user signal. The synthesizing unit receives the stage input signal and the user reproduction signals from the first interference estimating sections for the plurality of user signals, and outputs the residual signal for the next stage based on the stage input signal and the user reproduction signals. The last stage includes a second interference estimating section provided for each of the plurality of user signals. The second interference estimating section receives the residual signal outputted from the previous stage as the stage input signal, and receives the symbol replica from the previous stage. The second interference estimating section outputs a final user reproduction signal based on the symbol replica from the previous for the corresponding user signal and the stage input signal.
The coefficient output section may include a measuring unit provided for each of the plurality of user signals, for measuring an electric power of the corresponding user signal in the reception signal to outputs a user reception state signal associated with the corresponding user signal electric power, and a coefficient control unit provided for each of the plurality of user signals, for dynamically determining the weight coefficient based on the user reception stage signal. In this case, the measuring unit may measure the corresponding user signal electric power and an electric power of an interference signal for the corresponding user signal in the reception signal, and may determine a SIR ratio indicating a ratio of the corresponding user signal electric power to the interference signal electric power to output as the user state signal to the corresponding coefficient control unit. Alternatively, the measuring unit may measure the corresponding user signal electric power in the reception signal, and may determine the user state signal based on the corresponding user signal electric power to output the corresponding coefficient control unit.
In this case, the coefficient control unit dynamically determines a value of the weight coefficient as one of a plurality of values corresponding to the user state signal. The plurality of values are smaller than 1. Also, the coefficient control unit dynamically determines the weight coefficient value such that the weight coefficient value has a larger value when the user state signal indicates a larger value, and the weight coefficient value has a smaller value when the user state signal indicates a smaller value.
The first interference estimating section may include an interference estimating unit for receiving as the stage input signal and the symbol replica outputted from the previous stage when the stage is not the first stage, and for outputting the symbol replica for the next stage and a reproduction signal for the corresponding user signal based on the symbol replica received from the previous stage and the stage input signal, and a multiplier for multiplying the weight coefficient for the corresponding user signal from the coefficient output section and the reproduction signal from the interference estimating unit to output the user reproduction signal for the corresponding user signal.
The CDMA multi-user interference canceller may be contained in a receiving apparatus, a base station, or in a radio mobile station.
In order to achieve another aspect of the present invention, a method of receiving a plurality of user signals in an optimal state in a multi-user CDMA (Code Division Multiple Access) interference canceller, includes:
dynamically determining a weight coefficient based a reception state of each of the plurality of user signals of a reception signal; and
obtaining final user reproduction signals from the reception signal based on the weight coefficient,
wherein the obtaining step includes:
outputting a symbol replica for a next stage and a user reproduction signal for a corresponding user signal based on a symbol replica received from a previous stage when a current stage is not a first stage, a stage input signal and the weight coefficient for the corresponding user signal, the stage input signal being the reception signal when the stage is the first stage and a residual signal outputted from the previous stage when the stage is not the first stage; and
outputting the final user reproduction signal based on the symbol replica from the previous for the corresponding user signal and the stage input signal.